Kraft wood fibers for carboxymethyl cellulose

ABSTRACT

Disclosed is a method for producing kraft wood fiber having an alpha-cellulose content greater than 97% and a viscosity greater than 40 centipoise. The method involves prehydrolyzing hardwood chips with water, kraft cooking, bleaching and caustic treatment. The resulting pulp can be converted to carboxymethyl cellulose superabsorbents having improved properties, particularly a high “absorbency under load”.

[0001] This application is a continuation-in-part application of U.S.Ser. No. 09/467,579 filed Dec. 20, 1999, which claims priority from U.S.Serial No. 60/116,155 filed Dec. 30, 1998.

BACKGROUND OF THE INVENTION

[0002] The use of absorbent materials, commonly known assuperabsorbents, in disposable absorbent personal care products isknown. Such absorbent materials are generally employed in absorbentproducts such as diapers, training pants, adult incontinence products,feminine care products, and the like, in order to increase the absorbentcapacity of such products while reducing their overall bulk. Suchabsorbent materials are generally present in absorbent products in afibrous matrix, such as a matrix of wood pulp fluff. A matrix of woodpulp fluff generally has an absorbent capacity of about 6 grams ofliquid per gram of fluff. The absorbent materials described abovegenerally have an absorbent capacity of at least about 10, preferably ofabout 20, and often of up to 100 times their weight in water. Clearly,incorporation of such absorbent materials in personal care products canreduce the overall bulk while increasing the absorbent capacity of suchproducts.

[0003] A wide variety of materials have been described for use asabsorbent materials in such personal care products. Such materialsinclude natural-based materials such as agar, pectin, gums, carboxyalkylstarch, carboxyalkyl cellulose including carboxymethyl cellulose, andthe like, as well as synthetic materials such as polyacrylates,polyacrylamides, hydrolyzed polyacrylonitrile, and the like. While thenatural-based, absorbent materials are known for use in personal careproducts, they have not gained wide usage in such products. Such lack ofuse results, at least in part, from absorbent properties that areinferior compared to the synthetic absorbent materials such as thepolyacrylates. Specifically, many of the natural-based materials tend toform soft, gelatinous masses when swollen with a liquid. When employedin absorbent products, the presence of such soft gelatinous masses tendsto prevent the transport of liquid within the fibrous matrix in whichthe absorbent materials are incorporated. This phenomenon is known asgel-blocking. Once gel-blocking occurs, subsequent insults of liquidcannot be efficiently absorbed by the product, and the product tends toleak. Further, many of the natural-based materials exhibit poorabsorption properties, particularly when subjected to externalpressures. In contrast, the synthetic, absorbent materials are oftencapable of absorbing large quantities of liquid while maintaining agenerally stiff, non-gelatinous character. Accordingly, the synthetic,absorbent material can be incorporated in absorbent products whileminimizing the likelihood of gel-blocking.

[0004] Carboxyalkyl polysaccharide and carboxyalkyl cellulose materialsare well known in the art. Unfortunately, many known polysaccharide andcellulose materials do not possess absorptive properties comparable tomany of the synthetic, highly absorptive materials.

SUMMARY OF THE INVENTION

[0005] The invention is directed to a method for producing an absorbentcarboxyalkyl polysaccharide from a hardwood fiber pulp (kraft wood pulp)having an alpha-cellulose content greater than 97% and a viscositygreater than 40 centipoise (measured on a aqueous pulp via 0.5% CEDmethod). The method involves prehydrolyzing hardwood chips with water,at the end of this prehydrolysis step, no neutralization step wascarried out and the liquid was drained. Kraft cooking the wood chips,bleaching and then caustic treating of the chips. The resulting pulp canbe converted to a carboxyalkyl polysaccharide, preferably carboxyalkylcellulose, and most preferably carboxymethyl cellulose superabsorbentshaving improved properties, particularly a high Absorbency Under Load.

BRIEF DESCRIPTION OF THE DRAWING

[0006]FIG. 1 illustrates the apparatus for determining the AbsorbencyUnder Load values of an absorbent material.

DETAILED DESCRIPTION OF THE INVENTION

[0007] In one aspect, the present invention concerns a method forproducing kraft wood pulp having an alpha-cellulose content greater than97% and a viscosity greater than 40 centipoise (measured on an aqueouspulp via 0.5% CED method in accordance with Technical Association ofPulp and Paper Industry (TAPPI) test method T230 om-89). In anotheraspect, the wood pulp has a viscosity greater than about 42 centipoise.Preferably, the wood pulp comprises hardwood.

[0008] Although the basis for the wood pulp of the present invention ishardwood such as oaks, eucalyptuses, poplars, beeches, and aspens, awide variety of cellulosic fibers can be included in the process of thepresent invention. Illustrative cellulosic fibers include, but are notlimited to, wood and wood products, such as wood pulp fibers; non-woodypaper-making fibers from cotton, from straws and grasses, such as riceand esparto, from canes and reeds, such as bagasse, from bamboos, fromstalks with bast fibers, such as jute, flax, kenaf, cannabis, linen andramie, and from leaf fibers, such as abaca and sisal. It is alsopossible to use mixtures of one or more cellulosic fibers. Preferably,the cellulosic fiber used is from a wood source. Suitable wood sourcesinclude softwood sources such as pines, spruces, and firs, and hardwoodsources such as oaks, eucalyptuses, poplars, beeches, and aspens.

[0009] As used herein, the term “fiber” or “fibrous” is meant to referto a particulate material having a major dimension less than 10 mm,preferably less than 5 mm, often between about 0.1 mm and 3 mm, whereinthe length to diameter ratio (aspect ratio) of such particulate materialis greater than about 10. Conversely, a “nonfiber” or “nonfibrous”material is meant to refer to a particulate material wherein the lengthto diameter ratio of such particulate material is about 10 or less.

[0010] It is generally desired that the cellulosic fibers used herein bewettable. As used herein, the term “wettable” is meant to refer to afiber or material which exhibits a water in air contact angle of lessthan 90°. Suitably, the cellulosic fibers useful in the presentinvention exhibit a water in air contact angle between about 10° toabout 50° and more suitably between about 20° to about 30°. Suitably, awettable fiber refers to a fiber which exhibits a water in air contactangle of less than 90°, at a temperature between about 0°C. and lessthan about 100° C., and typically at ambient conditions, such as about23° to 28° C.

[0011] Suitable cellulosic fibers are those which are naturallywettable. However, naturally nonwettable fibers can also be used. It ispossible to treat the fiber surfaces by an appropriate method to renderthem more or less wettable. When surface treated fibers are employed,the surface treatment is desirably nonfugitive; that is, the surfacetreatment desirably does not wash off the surface of the fiber with thefirst liquid insult or contact. For the purposes of this application, asurface treatment on a generally nonwettable fiber will be considered tobe nonfugitive when a majority of the fibers demonstrate a water in aircontact angle of less than 90° for three consecutive contact anglemeasurements, with drying between each measurement. That is, the samefiber is subjected to three separate contact angle determinations and,if all three of the contact angle determinations indicate a contactangle of water in air of less than 90°, the surface treatment on thefiber will be considered to be nonfugitive. If the surface treatment isfugitive, the surface treatment will tend to wash off of the fiberduring the first contact angle measurement, thus exposing thenonwettable surface of the underlying fiber, and will demonstratesubsequent contact angle measurements greater than 90°.

[0012] Beneficial wettability agents include polyalkylene glycols, suchas polyethylene glycols. The wettability agent is used in an amountcomprising beneficially less than about 5 weight percent, suitably lessthan about 3 weight percent, and more suitably less than about 2 weightpercent, of the total weight of the fiber, material, or absorbentstructure being treated.

[0013] In the present invention, it is desired that the cellulosicfibers be used in a form wherein the cellulosic fibers have already beenrefined into a pulp. As such, the cellulosic fibers will besubstantially in the form of individual cellulosic fibers although suchindividual cellulosic fibers may be in an aggregate form such as a pulpsheet. The current process, then, is in contrast to known steamexplosion processes that generally treat cellulosic fibers that aretypically in the form of virgin wood chips or the like. Thus, thecurrent process is a post-pulping, cellulosic fiber modifying process ascompared to known steam explosion processes that are generally used forhigh-yield pulp manufacturing or waste-recycle processes.

[0014] The starting material for the method of the present inventionwill normally be wood chips in which the fibers are of a length suitablefor paper making.

[0015] Shavings could also be used but sawdust would be undesirableexcept as a minor part of the total furnish as the fibers are partiallycut. The chips should also, as is well known, be suitable in the senseof being free from bark and foreign matter.

[0016] It is desirable for the purposes of this invention that coarsechips be avoided. One problem with coarse chips is that cooking wouldnot be complete. It is best to use shredded or thin chips.

[0017] The method of the present invention involves treatment of thehardwood fibers or chips in four basic steps: prehydrolysis withoutneutralization of the prehydrolysis liquor at the end of prehydrolysis,kraft cooking, bleaching, and a caustic treatment. This treatmentproduces a kraft wood pulp having an alpha-cellulose content greaterthan 97% and a viscosity greater than 40 centipoise (when measured via0.5% CED method). Alpha-cellulose is the major component of wood andpaper pulp. It is that portion of cellulose that is insoluble in strongsodium hydroxide solution. Methods of determining the alpha content ofpulps are detailed in TAPPI Method T203 and ASTM D-588-42. Conventionaltreatments of hardwoods have been known to provide high alpha-cellulosekraft pulps, but the viscosity of these pulps is fairly low, generallybelow about 20 centipoise. It is unexpected to find out that the sametreatment of the softwood chips can have an alpha cellulose greater than97% but can not have a viscosity greater than 40 centipoise. This isprobably because the bonding among lignin, hemicellulose and celluloseis different for hardwood and softwood fibers. During the hydrolysis andcaustic treatment, removal of hemicellulose causes more damage to thecellulose structure of softwood fibers than the structure of hardwoodfibers.

Prehydrolysis

[0018] The first stage in the method of the present invention whereinprocedures can be utilized which improve the amount of hemicelluloseremoved from the lignocellulosic material while minimizing the amount ofdegradation of the cellulose, is the hydrolysis step. Wood chips inwater at liquor/wood ratio of 4/1 in a M/K digester were heated to 170degree C. with time to temperature of 60 minutes and time at temperatureof 20 minutes. The hydrolysis liquor was drained without neutralizationfrom the digester after the hydrolysis process is done.

[0019] The particular hydrolysis used in the method of this invention isdependent on the type of wood and the degree of removing ofhemicellulose. The hydrolysis has a great effect on the viscosity of thefinal high alpha-cellulose. In order to achieve desirable degree ofhydrolysis, H factor was used as a control tool. The definition of Hfactor can be found in a typical pulping book such as Pulping Processeswritten by Rydholm, published by Interscience Publishers, 1965.Basically, H Factor is a variable used in the Kraft cooking process tocombine the variables of temperature and time into a single variablerepresenting the extent of the cooking. For the hydrolysis process, theH Factor is used to characterize the degree of hydrolysis. For thepresent invention, the hydrolysis temperature and time should beadjusted to obtain an H-factor ranging from 300 to 1000, morespecifically about 700, with a corresponding minimum chip yield of 80%.The preferable chip yield is 90% in this invention. The yield is definedas the ratio of the resulted chip weight (oven dried basis) to theoriginal chip weight (oven dried basis).

Kraft Cooking

[0020] In the cooking step, the hydrolyzed chips were cooked in a M/Kdigester in a cooking liquor of sodium hydroxide and sodium sulfide at aliquor/wood ratio of 4/1 heated to 170 degrees C. for 35 to 60 minutes.The cooking liquor contained 15% effective alkaline and 25% sulfidity.The definition and calculation of effective alkaline and sulfidity canbe found in a typical pulping book such as Pulping Processes written byRydholm, published by Interscience Publishers, 1965. For the presentinvention, the effective alkaline can ranged from 10 to 20% andsulfidity from 15 to 40%. The H Factor is used to characterize thedegree of cooking. The H Factor used to obtain desirable pulp isdependent on the effective alkaline and sulfidity. For the presentinvention, the effective alkaline, sulfidity and H Factor should beadjusted to obtain an unbleached pulp with a minimum Kappa number of 5,in order to achieve desirable final product. The Kappa number is used torepresent the degree of lignin removal. The Kappa number was measuredaccording to TAPPI test method T236 cm-85.

Bleaching

[0021] In the next step, the unbleached fiber was subjected to ableaching process to remove residual lignin in a series of steps, usingselected combinations of chemical reactants. In the prior art, variouscombinations of chemical treatments have been suggested. Furthermore,individual treatment steps have been rearranged in an almost limitlessnumber of combinations and permutations. Therefore, in order to simplifythe explanation of the various bleaching processes, the use of lettercodes is conventionally employed in the combination to describe theparticular chemical reactants employed and the sequence of the steps ofthe process. The letter codes which will be used hereafter, whereappropriate, are as follows:

[0022] C=chlorination Reaction with elemental chlorine in acidic medium

[0023] E=Alkaline Extraction Dissolution of reaction product with NaOH

[0024] E(O)=Oxidative Alkaline Dissolution of reaction product with NaOHand Oxygen

[0025] D=Chlorine Dioxide Reaction with elemental chlorine Dioxide inacidic medium

[0026] P=Peroxide Reaction with peroxide in alkaline medium

[0027] O=Oxygen Reaction with elemental oxygen in alkaline medium

[0028] Z=Ozone Reaction with ozone

[0029] C/D A mixture of chlorine and chlorine dioxide

[0030] H=Hypochlorite Reaction with hypochlorite in an alkaline solution

[0031] For the present invention, many combinations such as D-E-D, C/D-E- D can be used to remove residual lignin and raise pulp brightness toat least about 70%, and preferable to 85% brightness except that theC(chlorination) and H(hypochlorite) will not be used because they willdegrade fiber and provide low viscosity.

Caustic Extraction

[0032]

[0033] The next step is a caustic extraction to further remove residualhemicellulose in the bleached fiber. The conditions of causticextraction is to treat the bleached fiber in a 6 to 12% caustic sodiumhydroxide solution for 10 to 100 minutes at 15 to 65 degree C. Aspecific conditions is to treat bleached fiber in the 7.5% causticsolution for 60 minutes at 25 degrees C. to obtain an alpha-cellulosecontent higher than 97%.

[0034] The treated wood pulp, prepared as described herein, can then beconverted to carboxyalkyl polysaccharide, preferably carboxyalkylcellulose, and most preferably carboxymethyl cellulose (CMC)superabsorbents by methods that are well known in the art. A preferredconversion process is described in U.S. Pat. No. 5,247,072 (Ning et al.)assigned to the assignee of the present invention, and incorporatedherein in its entirety. Preferably, the resulting carboxyalkylpolysaccharide, carboxyalkyl cellulose, or carboxymethyl cellulose has arelatively high molecular weight. It is generally most convenient toexpress the molecular weight of a carboxyalkyl cellulose in terms of itsviscosity in a 2.0 weight percent aqueous solution. Preferably thecarboxymethyl cellulose has a viscosity in a 2.0 weight percent aqueoussolution of from about 50 centipoise to about 80,000 centipoise,preferably from about 2,000 centipoise to about 80,000 centipoise, andmost preferably from about 20,000 centipoise to about 80,000 centipoise.

[0035] The carboxyalkyl cellulose will suitable have a pH within therange of from about 5.0 to about 11.0, beneficially from about 6.0 toabout 10.0, and preferably from about 6.5 to about 9. It is generallydesired that the carboxyalkyl cellulose has a generally neutralcharacter.

[0036] It is desired that the carboxyalkyl cellulose have an AbsorbencyUnder Load (AUL) of at least about 17, beneficially at least about 20,most beneficially at least about 24, and preferably at least about 27grams per gram.

[0037] It is unexpected to find that the CMC prepared from hardwoodfibers exhibited a substantially higher AUL than the CMC prepared fromsoftwood fibers.

EXAMPLES

[0038] The following examples present a non-limiting illustration of theinvention. All parts, percentages, ratios, etc. are given in weightunless otherwise noted.

Example 1

[0039] For Example 1, Terrace Bay Aspen wood chips were mixed with waterin a 4:1 water:wood ratio. This mixture was cooked for 60 minutes up toa temperature of about 170° C. and then held at 170° C. for 20 minutes.At the end of this prehydrolysis step, the liquid was drained withoutbeing neutralized. The prehydrolyzed aspen chips were mixed with analkali solution (14.5% effective alkali, 25% sulfidity) in a 4:1solution:wood chip ratio. This mixture was cooked for 60 minutes up to atemperature of about 170° C. and then held at 170° C. for 35 minutes. Atthe end of this kraft cooking step, the liquid was drained. For thebleaching step, the kraft cooked prehydrolyzed aspen chips weresubjected to three stages. The wood chips were diluted to 10% in anaqueous solution having 0.94% chlorine dioxide and were held at 135° F.(57° C.) for 60 minutes. A hot caustic extraction stage followed, wherethe chips were diluted to 10% in an aqueous solution having 1.5% sodiumhydroxide and were held at 160° F. for 70 minutes. The chlorine dioxidestage was repeated except that the woods chips were diluted to 10% in anaqueous solution having 0.6% chlorine dioxide and were held at 160° F.(71° C.) for 150 minutes. This was followed by a caustic treatment (coldcaustic extraction), where the chips were diluted to 10% in an aqueoussolution having 75% sodium hydroxide and were held at 77° F. (25° C.)for 60 minutes.

[0040] The resulting kraft pulp had an alpha-cellulose content of 97.8%and a viscosity of 42.9 centipoise.

Example 2

[0041] Example 2 was prepared as described for Example 1, except thatmixed Southern Hardwood chips were used instead of Aspen.

[0042] The resulting kraft pulp had an alpha-cellulose content of 98.7%and a viscosity of 40.5 centipoise.

Comparative Example A

[0043] Comparative Example A was a kraft wood pulp (available from ITTRayonier, under the trade name “Ultranier”). It is believed this was apine wood pulp.

Comparative Example B

[0044] Comparative Example B was a southern softwood kraft pulp(available from Kimberly-Clark Corporation under the designation “CR54southern softwood kraft pulp”).

[0045] The overall properties of Examples 1 and 2 and ComparativeExamples A and B are listed in Table 1, below. Table 1 includes thepercentage of alpha-cellulose, degree of polymerization in water (DPw),and viscosity in centipoise (measured via 0.5% CED method, TAPPI). TABLE1 Wood Pulp Properties % alpha- Example Wood Species cellulose DPwViscosity 1 Aspen 97.8 3593 42.9 2 Mixed Southern 98.7 3431 40.5Hardwood Comp. A assumed, Pine 98 1680 7 Comp. B Southern Pine 87.6 239622

[0046] The Absorbency Under Load (AUL) is a test which measures theability of an absorbent material to absorb a liquid (0.9 weight percentsolution of sodium chloride in distilled water) while under an appliedload or restraining force.

[0047] Referring to FIG. 1, the apparatus and method for determining AULwill be described. Shown is a perspective view of the apparatus inposition during a test. Shown is a laboratory jack 1 having anadjustable knob 2 for raising and lowering the platform 3. A laboratorystand 4 supports a spring 5 connected to a modified thickness meterprobe 6, which passes through the housing 7 of the meter, which isrigidly supported by the laboratory stand. A plastic sample cup 8, whichcontains the superabsorbent material sample to be tested, has aliquid-permeable bottom and rests within a Petri dish 9, which containsthe saline solution to be absorbed. A weight 10 rests on top of a spacerdisc (not visible) resting on top of the superabsorbent material sample(not visible).

[0048] The sample cup consists of a plastic cylinder having a 1 inchinside diameter and an outside diameter of 1.25 inch. The bottom of thesample cup is formed by adhering a 100 mesh metal screen having 150micron opening to the end of the cylinder by heating the screen abovethe melting point of the plastic and pressing the plastic cylinderagainst the hot screen to melt the plastic and bond the screen to theplastic cylinder.

[0049] The modified thickness meter used to measure the expansion of thesample while absorbing the saline solution is a Mitutoyo DigimaticIndicator, IDC Series 543, Model 543-180, having a range of 0-0.5 inchand an accuracy of 0.00005 inch (Mitutoyo Corporation, 31-19, Shiba5-chome, Minato-ku, Tokyo 108, Japan). As supplied from MitutoyoCorporation, the thickness meter contains a spring attached to the probewithin the meter housing. This spring is removed to provide a freefalling probe, which has a downward force of about 27 grams. Inaddition, the cap over the top of the probe located on the top of themeter housing is also removed to enable attachment of the probe to thesuspension spring 5 (available from McMaster-Carr Supply Co., Chicago,Ill., Item No. 964OK41), which serves to counter or reduce the downwardforce of the probe to about 1 gram, ±0.5 gram. A wire hook can be gluedto the top of the probe for attachment to the suspension spring. Thebottom tip of the probe is also provided with an extension needle(Mitutoyo Corporation, Part No.131279) to enable the probe to beinserted into the sample cup.

[0050] To carry out the test, a 0.160 gram sample of the absorbentmaterial, which has been sieved to a particle size between 300 and 600microns, is placed into the sample cup. The sample is then covered witha plastic spacer disc, weighing 4.4 grams, which is slightly smallerthan the inside diameter of the sample cup and serves to protect thesample from being disturbed during the test. The 100 grams weight isthen placed on top of the spacer disc, thereby applying a load of 0.3pounds per square inch. The sample cup is placed in the Petri dish onthe platform of the laboratory jack raised up until it contacts the tipof the probe. The meter is zeroed. A sufficient amount of salinesolution is added to the Petri dish (50-100 milliliters) to begin thetest. The distance the weight is raised by the expanding sample as itabsorbs the saline solution is measured by the probe.

[0051] This distance, multiplied by the cross-sectional area inside thesample cup, is a measure of the expansion volume of the sample due toabsorption. Factoring in the density of the saline solution and theweight of the sample, the amount of saline solution absorbed is readilycalculated. The weight of saline solution absorbed after 60 minutes isthe AUL value, expressed as grams saline solution absorbed per gram ofabsorbent. If desired, the readings of the modified thickness meter canbe continuously input to a computer (Mitutoyo Digimatic MiniprocessorDP-2 DX) to make the calculations and provide AUL readings. As across-check, the AUL can also be determined by determining the weightdifference between the sample cup before and after the test, the weightdifference being the amount of solution absorbed by the sample.

[0052] Examples 1 and 2 and Comparative Examples A and B were convertedinto carboxymethyl cellulose (CMC) superabsorbents as taught by U.S.Pat. No. 5,247,072. More specifically, 15 grams of cellulose (0.0943mole) was first immersed in 400 ml isopropanol in a reaction kettleequipped with a mechanical stirrer, an inert gas inlet and a temperaturecontrol probe. 8.31 g (0.208 mole) NaOH dissolved in 35 ml watersolution (if the starting cellulose was in wet pulp form with about 30%consistency, no water is needed, adding in NaOH pellets directly) wasadded. The pulp slurry was stirred for half an hour at room temperature(1 hour if dry NaOH is used). 8.9 g (0.0945 mole) of Chloroacetic acid(CAA) was then added and the temperature raised to 60 C under stirring.The reaction continued for three hours at 60C. After that the slurry wasfiltered, the product washed twice with 70:30 (by volume) mixturesolvent of methanol and water (400 ml solution for each wash). The pH ofthe slurry in washing liquid was adjusted to around 7.4 with acetic acidduring the first wash cycle. Finally the CMC fibers was washed one moretime with 100% methanol and allowed to dry at 50C. This will generallyyield about 21 grams of dry CMC with a typical degree of substitution(D.S.) at 0.9.

[0053] Procedures for Preparing CMC-SAP:

[0054] The CMC fibers were then dissolved in water to make a 2%solution, dried at 50C and ground into granules. Particle sizes ranging300-600 microns were collected for thermo-curing and absorbency test.This part of the procedure is in accordance with U.S. Pat. No.5,247,072, previously incorporated by reference. The resultingsuperabsorbents were tested for Absorbency Under Load, as described bythe above test procedure. The AUL results, as a function of curing time,are given in Table 2, below. TABLE 2 AUL results, reported in grams/gramTime (minutes) Example 0 15 30 45 60 90 120 1 21.4 — — — 23 22.2 21.8 222.7 22.2 21.2 21.8 21.5 21.6 20.8 Comp. A 16.5 — 19.4 19.5 19.6 19.419.8 Comp. B 12 13.2 13.4 13.7 13.3 13.4 13.5

[0055] It will be appreciated that the foregoing description andexamples, given for purposes of illustration, are not to be construed aslimiting the scope of this invention, which is defined by the followingclaims.

We claim:
 1. A method for producing an absorbent carboxyalkylpolysaccharide composition comprising (a) prehydrolyzing hardwood chipswith water; (b) kraft cooking the prehydrolyzed hardwood chips to reducethe hardwood chips to fibers; (c) bleaching the cooked hardwood fibers;(d) treating the bleached hardwood fibers with caustic to form a treatedhardwood fiber pulp having an alpha-cellulose content greater than 97%and a viscosity greater than 30 centipoise; and (e) converting thetreated hardwood fiber pulp to a carboxyalkyl polysaccharide having anAUL of about 20 or greater.
 2. The method of claim 1 wherein thehardwood chips are prehydrolyzed at an H factor of from about 300 toabout 1000 with a corresponding maximum chip yield of about 80 percent.3. The method of claim 1 wherein the hardwood chips are prehydrolyzed atan H factor of from about 500 to about
 800. 4. The method of claim 1wherein the hardwood chips are prehydrolyzed at an H factor of about 600to about
 700. 5. The method of claim 1 wherein the prehydrolyzedhardwood chips were cooked in a cooking liquid of sodium hydroxide andsodium sulfide to obtain an unbleached pulp with a Kappa number of 5 orgreater.
 6. The method of claim 1 wherein the caustic treatment of thebleached wood fibers is carried out with a sodium hydroxide solution atfrom about 15 to about 65 degrees C. from about 10- to about 100minutes.
 7. The method of claim 1 wherein the AUL is from 20 to about25.
 8. The method of claim 1 wherein the viscosity of the treatedhardwood fiber pulp is about 40 centipoise or greater.
 9. The method ofclaim 1 wherein the water used to hydrolyze the hardwood chips of step(a) is drained from the hardwood chips without neutralization.